1. Field of the Invention
This invention relates generally to in-line monitoring of via/contact etching process in semiconductor device fabrication by using a scanning electron microscope (SEM), and more particularly to methods and devices for determining whether via/contact holes are over or under etched in the process of fabricating a semiconductor device.
2. Description of the Prior Art
Very large-scale integrated (VLSI) circuits rely on via/contact holes (as well as trenches) for electrically interconnecting devices of different layers, interconnecting a layer to an underlying substrate, or interconnecting a layer to another layer. The electrical defects associated with the deep (high aspect ratio) sub-micron via/contact holes take up a significant part of the total yield loss as the current technology approaches to the 0.1 μm node. It is, therefore, essential to ensure that the etching process for creating such via/contact holes is optimized and in-line controlled within the process window, so that the potential via/contact failure due to either process parameters shifting or wafer-to-wafer dielectric thickness variation can be identified and avoided in the early steps.
Because of the nonuniformity in the etch rate, and the fact that the film itself may be of nonuniform thickness across the wafer or from wafer to wafer, a certain amount of over-etching is done to ensure that complete etching is achieved everywhere on the wafer, and appropriate electrical contact is obtained. This is often 10–20% over-etching in terms of time past the endpoint point. Even more over-etching (as much as 50%) may be required when anisotropic processing is done over non-planar topography. However, as the technology shrinks into the 0.10 μm mode, the thickness of over-etch margins have dropped drastically. Excessive over-etch of contact holes will cause the thin metal silicide layer on top of a drain/source region to be diminished due to the limited selectivity of the etch process. Also important, there is high probability that the contact penetrates the shallow pn-junction beneath the drain/source that leads to high leakage current. For via etch processes aiming at open dielectric barriers over the lower metal level, it is also necessary to avoid excessive dielectric barrier over-etch; otherwise, copper is exposed and sputtered during the over-etch step, potentially compromising device reliability.
The integrity of via/contacts can be validated by measuring the resistance of long chains connecting thousands of vias/contacts in series with each other and located in the scribe lines or in test chips on the wafer. These via/contact chains pass over vanous topographies. A current is forced through these long chains, and the measured voltage is a measure of the average contact resistance. These structures are used to monitor the via/contact as functional processing conditions and structures, and to measure lot-to-lot variation. A high value of resistance in these structures could indicate a problem with under-etch, over-etch, andlor etch residue, but may also be causes of poor metal deposition, voids in contact region, or other problems incurred in subsequent processes. In addition, this test cannot be performed before completion of the conductive wiring chain. This increases the manufacturing cost.
An electron beam inspection system, or in its simplest form, a conventional scanning electron microscope (SEM), has been proven to be a powerful tool for imaging electrical defects such as via/contact short. As the primary electron beam scans over the inspection area, low energy secondary electrons (SE) (˜5 eV) will be generated from the surface and collected by the SE detector to form an image. Due to the differences in SE yields of the involved materials or the abnormal electrical conductivity of the defect portions, the inspected surface will be unevenly charged positively and/or negatively. Negatively charged surfaces tends to produce more SE to the signal detector, thus its appearance is relatively brighter, while a positively charged surface attracts more SE and thus appears relatively darker. This is the so-called voltage contrast (VC). VC can be used roughly to divide via/contact holes into the categories of under-etch or over-etch. However, it lacks the sensitivity to the level of under-etch or over-etch, thus is not suitable for process monitoring.